In recent years, much attention has been paid to a thermodynamic cycle conventionally known as the reversed Stirling cycle, and mush research has been done on Stirling cycle refrigerators exploiting it. A Stirling cycle refrigerator is an external combustion engine that has a heat absorber portion and a heat rejector portion, that adopts an environmentally harmless gas, such as helium, nitrogen, or hydrogen, as a working medium, and that is so structured that cold (cryogenic heat) is obtained in the heat absorber portion.
However, most Stirling cycle refrigerators in practical use offer cooling performance as low as a few watts or lower and are designed for cryogenic applications. That is, no Stirling cycle refrigerators have ever been in practical use as refrigerators having cooling performance on the order of several hundred watts, the rank of cooling performance most sought-after in refrigerators for home and business use.
Now, taking up as an example a typical free-piston-type reversed Stirling cycle engine (hereinafter referred to as the reversed Stirling cycle engine) shown in FIG. 11, the problems that remain to be tackled in it will be explained. In terms of its external appearance, the reversed Stirling cycle engine 186 is composed of a pressure vessel 174, a warm head 182, and a cold head 183. Inside, a working space 178 is secured, which is filled with a working medium, such as helium.
Inside a cylinder that constitutes part of the pressure vessel 174, a piston 171 and a displacer 172 are arranged coaxially so as to be slidable along the inner wall surface of the cylinder. A rod 175 has one end fixed to the displacer 172, penetrates the piston 171 at its center, and has the other end resiliently supported on the pressure vessel 174 by a displacer support spring 177. The piston 171 is resiliently supported on the pressure vessel 174 by a piston support spring 176.
The working space 178 has a compression space 181a and an expansion space 181b. The piston is made to reciprocate axially inside the compression space 181a with a predetermined period by a piston driving member 180, such as a linear motor, housed in a back space 179. The piston support spring 176 serves to stabilize the period of the piston 171 by keeping it substantially constant once the piston 171 starts reciprocating.
A regenerator 173, on the one hand, collects cold from the working medium moving from the expansion space 181b to the compression space 181a and accumulates the cold. The regenerator 173, on the other hand, transfers the accumulated cold to the working medium moving from the compression space 181a to the expansion space 181b and thereby cools the working medium. In this structure, the temperature of the working medium moving between the compression space 181a and the expansion space 181b varies greatly as a result of the working medium being compressed and expanded, and this makes it possible to obtain cold efficiently from the reversed Stirling cycle engine 186.
When compressed in the compression space 181a, the working medium passes through the regenerator 173 and moves toward the expansion space 181b. This causes the displacer 172 to reciprocate axially with the same period as the piston 171 and with a predetermined phase difference kept relative to the piston 171. As a result, in the expansion space 181b, the working medium is repeatedly compressed and expanded in such a way as to exhibit sinusoidal pressure variation.
The displacer support spring 177 has its spring constant and other parameters so set that, once the displacer 172 starts reciprocating, it stabilizes the period of the displacer 172 by keeping it equal to that of the piston 171 and keeps the phase difference relative to the piston 171 constant. Expanded in the expansion space 181b, the working medium becomes cooler and thus cools the cold head 183, absorbing heat from outside. On the other hand, compressed in the compression space 181a, the working medium becomes warmer and thus warms the warm head 182 provided at the compression space 181a side end of the regenerator 173, rejecting heat to outside.
To reduce the vibration of the reversed Stirling cycle engine 186 that occurs mainly axially as a result of the reciprocating movement of the piston 171 and the displacer 172, a vibration damping mechanism, composed of a vibration damping spring 184 and a block 185 made of a metal or the like and having a predetermined weight, is provided at that end of the reversed Stirling cycle engine 186 which faces away from the cold head 183.
This reversed Stirling cycle engine 186, however, generally offers cooling performance as low as, for example, several tens to a hundred and several tens of watts if its size is limited to be comparable with that of refrigeration cycle apparatus of a vapor compression type as are used in modern refrigerators for home use. Thus, to obtain cooling performance on the order of several hundred watts most sought-after on the market of home-use products, the following measures need to be taken.
First, the volume of the working medium that is compressed and expanded is increased by giving the piston 171 and the displacer 172 larger diameters. Second, the strokes (amplitudes) over which the piston 171 and the displacer 172 are made to reciprocate are increased. Third and last, the frequency at which the piston 171 and the displacer 172 are made to reciprocate is increased.
However, the first measure has the following disadvantages. To give the piston 171 and the displacer 172 larger diameters, it is necessary to give the pressure vessel 174, in which they are housed, a larger external diameter. This permits the piston 171 and the displacer 172 to stagger more in the direction perpendicular to the axis as they slide inside the cylinder, and thus makes the piston 171 and the 172 more likely to make contact with the pressure vessel 174. If such contact occurs frequently, or if an external force is unexpectedly applied to the pressure vessel 174, the breaking stress that occurs as a counteracting force is accordingly larger. This leads to trouble, such as a crack in the pressure vessel 174.
Thus, it is necessary to make the entire pressure vessel 174, including the warm head 182 and the cold head 183, somewhat thicker. Making these thicker, however, increases the weight and thus the costs of the engine as a whole.
Moreover, making the warm head 182 thicker increases heat resistance, in particular in radial directions, and thus makes it more difficult for heat to conduct to the heat exchanger for heat rejection (not shown) provided around the warm head 182, while reducing heat resistance in the axial direction and thereby making it easier for heat to conduct from the warm head 182 to the cold head 183 through the components constituting the pressure vessel 174 and the like. This lowers the efficiency of the reversed Stirling cycle engine 186. Furthermore, the larger the piston 171 and the displacer 172, the heavier the reversed Stirling cycle engine 186 as a whole, increasing the burden on the power source, the support springs, and the like.
The second measure has the following disadvantages. Since the piston 171 is resiliently supported on the pressure vessel 174 by the piston support spring 176 and the displacer 172 is resiliently supported on the pressure vessel 174 by the displacer support spring 177, increasing the strokes over which they are made to reciprocate axially increases the strokes of the support springs 176 and 177.
As their strokes are increased, the support springs 176 and 177 become more likely to receive excessive force and thus more prone to damage, such as breakage. This eventually leads to lower performance of the reversed Stirling cycle engine 186. Furthermore, as the strokes of the support springs 176 and 177 are increased, the piston driving member 180, such as a linear motor, is inevitably required to exert higher power. This increases electric power consumption, contrary to energy saving.
The third measure has the following disadvantages. The spring constants of the support springs 176 and 177 are proportional to the square of the frequency at which they are used. Thus, if an attempt is made, for example, to double the performance of a Stirling cycle refrigerator by increasing its operation frequency, the resulting vibration puts at least four times the load on the support springs 176 and 177. Given the constraints on the materials of the support springs 176 and 177, there is a risk of their elastic limit being exceeded.
Because of the problems explained above, in a home-use refrigerator that is required to offer cooling performance on the order of several hundred watts, with a single unit of the free-piston-type reversed Stirling cycle engine 186 described above, it is not possible to obtain sufficiently high cooling performance. That is, it is not possible to achieve the desired cooling effect in the space to be cooled. Moreover, the vibration damping mechanism described above is indispensable in practical terms. This increases the dimensions and weight of the reversed Stirling cycle engine 186, and thus increases its manufacturing costs accordingly.